1. Field of the Invention
The present invention relates to an optical communication system in which one optical communication apparatus and another are coupled to each other by an optical cable and the optical communication apparatus and the optical cable that constitute this optical communication system.
2. Description of Related Art
In an optical communication system in which optical communication apparatus that is equipped with a light emitting element for outputting an optical signal and a light receiving element for receiving the optical signal is coupled to another by an optical fiber cable, the optical fiber cable can be attached to and detached from the optical communication apparatus.
In the optical communication apparatus, as far as power is applied to them, for example, the light emitting element continues to emit light. Therefore, even if the optical fiber cable is not coupled to the optical communication apparatus, the light emitting element continues to emit light in the optical communication apparatus.
A technology for solving this problem is known which attempts to reduce power consumption by stopping emission of light from the light emitting element when the optical fiber cable is not coupled (see Japanese Patent Application Laid-Open No. 2000-340306).
FIGS. 1A–1D are conceptual diagrams each for showing a configuration of a conventional optical communication system, which incorporates a mechanism capable of stopping emission of light from light emitting element when an optical cable is not coupled.
As shown in FIG. 1A, a conventional optical communication system 101 has such a configuration that an optical communication apparatus 102 and an optical communication apparatus 103 are coupled to each other by an optical fiber cable 104. The one optical communication apparatus 102 is equipped with a light emitting element 105 such as a laser diode. The other optical communication apparatus 103, on the other hand, is equipped with a light receiving element 106 such as a photodiode 106.
The optical fiber cable 104 is provided with a plug 107 at each of its opposite ends so that it can be attached to and detached from each optical communication apparatus 102, 103. The optical communication apparatus 102 equipped with the light emitting element 105 is also equipped with a detection circuit 108 for detecting whether this plug 107 is connected to it. This detection circuit 108 is arranged to be switched between, for example, a conductive state and a nonconductive state in accordance with whether the plug 107 is connected or not. In addition, it is equipped with a monitor 109 for monitoring whether the detection circuit 108 is conductive so that in accordance with a result of monitoring by this monitor 109, a control system, not shown, may control emission of light from the light emitting element 105.
In this conventional optical communication system 101, if the optical fiber cable 104 is extracted from the optical communication apparatus 102, shown in FIG. 1B, the detection circuit 108 is switched, for example, from the conductive state to the nonconductive state. The monitor 109 detects a change in the state, thereby enabling stopping emission of light from the light emitting element 105.
On the contrary, if the optical fiber cable 104 is extracted from the optical communication apparatus 103, shown in FIG. 1C or if the optical fiber cable 104 is broken, shown in FIG. 1D, the detection circuit 108 stays unchanged in state of conduction. Thus, it is impossible to stop emission of light from the light emitting element 105.
FIGS. 2A–2C are conceptual diagrams each for showing a configuration of another conventional optical communication system which has realized bidirectional communication. This conventional optical communication system 111 has realized single-core bidirectional optical communication and has such a configuration that an optical communication apparatus 112 and an optical communication apparatus 113 are coupled to each other by an optical fiber cable 114 as shown in FIG. 2A. The one optical communication apparatus 112 is equipped with a light emitting element 115a such as a laser diode and a light receiving element 116a such as a photodiode. The other optical communication apparatus 113, on the other hand, is equipped with a light emitting element 115b such as a laser diode and a light receiving element 116b such as a photodiode.
The optical fiber cable 114 is provided with a plug 117 at each of its opposite ends so that it can be attached to and detached from each optical communication apparatus 112, 113. The optical communication apparatus 112 is equipped with a detection circuit 118a for detecting whether this plug 117 is connected to it. This detection circuit 118a is arranged to be switched between, for example, the conductive state and the nonconductive state in accordance with whether the plug 117 is connected to it or not. In addition, it is equipped with a monitor 119a for monitoring whether the detection circuit 118a is conductive. In accordance with a result of monitoring by this monitor 119a, a control system, not shown, may control emission of light from the light emitting element 115a. 
The optical communication apparatus 113, on the other hand, is equipped with a detection circuit 118b for detecting whether the plug 117 is connected to it. This detection circuit 118b is arranged to be switched between, for example, the conductive state and the nonconductive state in accordance with whether the plug 117 is connected to it or not. In addition, it is equipped with a monitor 119b for monitoring whether the detection circuit 118b is conductive. In accordance with a result of monitoring by this monitor 119b, a control system, not shown, may control emission of light from the light emitting element 115b. 
In this conventional optical communication system 111, if the optical fiber cable 114 is extracted from the optical communication apparatus 112, shown in FIG. 2B, the detection circuit 118a is switched, for example, from the conductive state to the nonconductive state. The monitor 119a detects a change in the state, thereby enabling stopping emission of light from the light emitting element 115a. 
However, the detection circuit 118b in the optical communication apparatus 113 to which the optical fiber cable 114 is coupled stays unchanged in state of conduction, so that it is impossible to stop emission of light from the light emitting element 115b. This holds true also with a case where the optical fiber cable 114 is extracted from the optical communication apparatus 113, not shown though.
Furthermore, if the optical fiber cable 114 is broken as shown in FIG. 2C, neither the detection circuit 118a in the optical communication apparatus 112 nor the detection circuit 118b in the optical communication apparatus 113 is changed in state of conduction. Emission of light cannot be stopped either from the light emitting element 115a or 115b. 
As described above, in the conventional optical communication system, if an optical fiber cable is extracted from an optical communication apparatus itself equipped with a light emitting element, the system can detect the extraction to stop emission of light from the light emitting element. However, if the optical fiber cable is extracted from the other optical communication apparatus or if it is broken, the extraction cannot be detected. This has brought about a problem that if the optical fiber cable is extracted from the other optical communication apparatus or if it is broken, it is impossible to stop emission of light from the light emitting element.